I'm assuming a diesel is a diesel as i've been around them for 30 years. Delvac 15W40 works good in the trucks, has excellent soot dispersion properties. I've got one unit I run 50,000 miles between oil changes and the oil analysis comes back good. I'll feel safe with delvac Sythetic. Just have to play it by ear.! BTW, three cars coming in, a million dumb questions~